1. The Field of the Invention
The invention relates to a radiation-selective absorber coating, especially for an absorber tube of a parabolic collector. The invention also relates to an absorber tube with such a radiation-selective coating. The invention also relates to a process for producing it and to a process for operating a parabolic collector using such absorber tubes.
2. The Description of the Related Art
Customary absorber coatings consist of a layer which reflects in the infrared region and is applied to a substrate, especially a metal tube, a cermet layer which possesses a high level of absorption in the region of the solar spectrum, and a top layer which is applied to the cermet layer, is referred to as an antireflection layer and, owing to the high refractive index of the cermet layer, is intended to reduce surface reflection on the cermet layer.
The fundamental aim is to achieve a maximum energy yield. Since the energy yield depends, among other factors, on the absorption coefficient α and the emission coefficient ε, the aim is always a high absorption (α>95%) and a low emission (ε<10%) of the absorber coating.
In addition, the efficiency of the collector is determined by the temperature at which it is operated. From this point of view, a maximum temperature is desired. Contrary to this, the lifetime of the layer system of the absorber coating, however, decreases with increasing operating temperature owing to ageing and/or diffusion processes, as a result of which, for example, the absorption property of the cermet layer and the reflection property of the layer which reflects in the infrared range can decline significantly.
DE 101 50 738 C1 therefore describes a radiation-selective absorber coating which is said not to exhibit any relevant color change and hence any ageing. This is achieved by virtue of a particular oxygen volume flow being established in the course of application of a third layer consisting of aluminium and aluminium oxide. A final layer of Al2O3 is applied to this layer.
U.S. Pat. No. 5,523,132 discloses an absorber coating in which a plurality of cermet layers are provided, which differ in the metal content and hence in the refractive index. By virtue of a plurality of absorption maxima being created at different wavelengths, a better match to the solar spectrum should be achieved. Anti-diffusion layers can be provided between the cermet layer and the layer which reflects in the IR region or between the cermet layer and the antireflection layer, though no statements are made about material and layer thickness.
DE 10 2004 010 689 B3 discloses an absorber comprising a radiation-selective absorber coating which comprises a metal substrate, a diffusion barrier layer, a metal reflection layer, a cermet layer and an antireflection layer. The diffusion barrier layer is an oxide layer which consists of oxidized components of the metal substrate.
Michael Lanxner and Zvi Elgat in SPIE Vol. 1272 Optical Materials Technology for Energy Efficiency and Solar Energy Conversion IX (1990), page 240 to 249, with the title “Solar selective absorber coating for high service temperatures, produced by plasma sputtering” describe an absorber coating which is applied to a steel substrate and comprises an antireflection layer of SiO2, a cermet layer composed of an Mo/Al2O3 composition and a layer of molybdenum which reflects in the infrared region, a diffusion barrier of Al2O3 being arranged between the layer which reflects in the infrared region and the substrate.
For the layers which reflect in the infrared region, molybdenum is typically used. However, the reflection properties of a molybdenum layer are not optimal, and so it is desirable to use better-reflecting materials.
The operating temperature range of known absorber tubes is from 300 to 400° C. under reduced pressure. For the reasons above, there is a fundamental drive to further increase the operating temperature, but without impairing, for example, the absorption properties of the cermet layer and the reflection properties of the layer which reflects in the infrared region.
C. E. Kennedy, “Review of Mid- to High-Temperature Solar Selective Absorber Materials”, Technical Report of the National Renewable Energy Laboratory, July 2002 edition, summarizes such efforts. This discloses a layer structure composed of a ZrOxNy or ZrCxNy absorption layer and a layer of Ag or Al which reflects in the IR region, which, by virtue of introduction of an Al2O3 diffusion barrier layer, has an improved thermal stability under air. In addition, it has been found that the thermal stability of the infrared reflection layer under reduced pressure can be improved by introducing a diffusion barrier layer below this layer. For this barrier layer, Cr2O3, Al2O3 or SiO2 are proposed as the layer material. It is hoped that this will achieve stability of the silver reflection layer up to 500° C.
However, this does not put an end to the drive toward more durable layers with simultaneously improved absorption and emission.